Published patent application WO2014/097089, which is incorporated by reference, discloses a lighting unit and a luminaire for illumination and for purifying air. The lighting unit comprises a photocatalytic volume through which, in use, air flows. The photocatalytic volume comprises photocatalytic material that is configured to be a catalyst in photoreactions between gasses in the air flow under the influence of light that impinges on the material. The lighting unit at least comprises a solid state light emitter that emits light towards the photocatalytic volume and the emitted light at least comprises light that assists the photoreactions between the gasses in the air flow.
The photocatalytic volume of the lighting unit of the cited document comprises open spaces, such as pores or holes, through which the air flow is able to flow from one side of the photocatalytic volume to another side of the photocatalytic volume. In many typical use cases it might be that inside these open spaces, or at the entry windows of the open spaces, bacteria colonies start to grow because such bacteria are delivered from the ambient onto the surfaces of the photocatalytic volume by the air flow and because there is quite often a relatively good climate for such bacteria to grow in the lighting unit. In relatively humid and/or warm climates, water may condensate in the lighting unit and then the water and the relatively high temperature provide a very good climate for the bacteria to grow. If too many colonies of bacteria are formed, no effective air purification is possible anymore because the photocatalytic volume may become less permeable for air and the bacteria or debris of the bacteria may provide unpleasant compounds (in the gaseous phase) to the air that flows through the photocatalytic volume. Furthermore, the large number of bacteria colonies may induce health risks because the gasses released by the bacteria or the debris may be poisonous and/or more bacteria may be present in the air flow that leaves the lighting unit of the cited patent application than there are present in the flow that enters the lighting unit.